The introduction of digital communications into the telephone network has greatly increased the available signal bandwidth. A corresponding reduction in the amount of cable required to carry a number of telephone conversations carried by analog telephony methods was also realized. The massive increase in the reliance and demand of telephone systems in today's society has driven the need for this reduction in cable and increased signal bandwidth.
A number of different technologies have been devised to facilitate the advance of digital communications. Various signaling technologies have used frequency division multiplexing (FDM) to segment the available bandwidth. Separate signals are carried in each segment using digital signaling derived through a digital representation of an analog waveform. Time-division multiplexing (TDM) has been used with T-1 and other transmission facilities, where each of a multiplicity of channels gets an interleaved time segment in order that all of the channels share the transmission medium equally.
A relatively newer and emerging field of telecommunications is the integrated services digital network (ISDN), which is a standard established by the International Telecommunications Union-Telecommunications (ITU-T-T) Standardization Sector. ISDN integrates computer and communications technologies to provide a common, worldwide, digital network. ISDN telephone lines use digital communications protocols allowing digital connections at up to 128 Kbps and as many as three separate conversations at the same time through the same line as the twisted-pair copper telephone line that traditionally carried only one voice, or one computer/fax communication.
There are two basic types of ISDN service. Primary rate interface (PRI) is intended for users with large bandwidth capacity requirements and provides up to 23 64-Kbps data channels (B-channels) and 1 64-Kbps signaling channel (D-channel) in the U.S. Basic rate interface (BRI) consists of 2 64-Kbps B-channels and 116-Kbps D-channel for a total of 144 Kbps. This basic service is intended to meet the needs of most individual users.
One of the limiting factors in the deployment of the basic rate ISDN digital subscriber line is that the standard physical layer transceivers have a maximum loop reach of 18,000 feet of AWG26 cable. This is due to the attenuation of the signal strength over a distance caused by the electrical properties of the line itself, including the resistance, capacitance, inductance, and leakage conductance. Due to subscriber loop reach limitations, either additional subscriber loops are required to reach more remote subscribers, or repeaters are necessary.
Analog telephony transmission systems faced subscriber loop problems, as it was discovered that voice signals would travel with reduced loss, and with greater fidelity, if the distributed inductance of the line was increased without adversely changing the other distributed circuit coefficients. A practical alternative used in lieu of tackling the difficult process of increasing the uniformly distributed inductance of a line was to insert lumped inductance coils at various intervals along the line. Loop design balanced cost against transmission quality, and loading permitted the economy, in long telephone lines, of using smaller gauge copper wires than would otherwise have been needed to give the same electrical efficiency and quality of transmission.
Digital communications, however, has transmission characteristics different from those of existing analog telephony. This difference has resulted in a generally implemented ISDN communications installation procedure across the existing twisted-pair telephone wires, including removing all load coils from the line as basic rate ISDN service is installed. As previously indicated, BRI ISDN local loops are currently limited to a maximum loop reach of approximately 18,000 feet of AWG26 cable, which results in a large number of subscribers who are not accessible within the normal local subscriber loop.
One solution currently implemented in digital transmission lines is the use of repeater amplifiers in the long subscriber loops. Repeater amplifiers, however, are implemented at a cost of thousands of dollars per line. Furthermore, repeater amplifiers include active electronic devices, which decreases the field reliability of service and increases the cost of service maintenance. Another approach to lengthen the subscriber loop involves the use of non-standard transceivers at each end of the long loop, resulting in yet another costly installation.
Accordingly, there is a need in the digital communications industry for a transmission arrangement to increase the maximum subscriber loop reach which is inexpensive, easy to implement, and reliable. The present invention provides a system and method for reducing the loop loss on digital subscriber loops having component costs of only tens of dollars per subscriber line. Therefore, the present invention provides a low cost and reliable solution for reaching approximately 95 percent or more of potential digital communications customers without the use of active electronics coupled to the subscriber line. The present invention overcomes the aforementioned problems, and provides these and other advantages over the prior art.